Desired magnetic properties of steels used for motor and transformer laminations are low core loss and high permeability. Those steels which are stress relief annealed after punching should have mechanical properties which minimize distortion, warpage and delamination during the annealing of the lamination stacks.
Continuously annealed silicon steels are conventionally used for motors, transformers, generators and similar electrical products. Continuously annealed silicon steels can be processed by techniques well known in the art to obtain low core loss and high permeability. Since the steels are substantially free of strain, they can be used in the as-punched condition (commonly referred to as fully processed steels) or can be finally annealed by the electrical apparatus manufacturer after punching of the laminations (commonly referred to as semi-processed steels) to produce the desired magnetic properties with little danger of delamination, warpage, or distortion. Continuous annealing processing requires the electrical steel sheet manufacturer to have a continuous annealing facility. The equipment for a continuous annealing facility requires a capital expenditure of many millions of dollars.
To avoid a continuous annealing operation, practices have been developed to produce cold rolled motor lamination steel by normal cold rolled sheet processing including batch annealing followed by temper rolling. Continuous annealing processes differ in many respects from normal cold rolled sheet processing. For example, continuous annealing subjects the coil to uniform annealing conditions, whereas batch annealing does not.
In addition, a continuously annealed product does not require temper rolling for flattening, because when steel is continuously annealed it has little strain imparted to it from the annealing process. Although batch annealing facilities use much lower cost equipment than continuous annealing facilities, batch annealing facilities are not able to produce a sufficiently flat product without temper rolling. Strain imparted by temper rolling leads to delamination and warpage problems of motor lamination steel. At the present time, delamination and warpage resulting from this strain is a serious concern to such customers.
Steel can be produced to have either "oriented" grains, or "non-oriented" grains. Grain oriented silicon steels are characterized by very high permeability and low core loss in the rolling direction. For example, at 1.5 Telso ("T") and 60 Hertz ("Hz"), a 0.012 inch thickness strip may have a permeability in the rolling direction of 28,000 Gauss/Oersted ("G/Oe") and a core loss in the rolling direction of 0.58 Watts/pound ("W/lb").
Grain oriented silicon steels have superior magnetic properties in the rolling direction as a result of a so-called Goss texture, i.e., a {110}&lt;001&gt; orientation as defined by the Miller crystallographic indexing system. Steel having a Goss texture is magnetically anisotropic, i.e., it has a sheet-plane variation of permeability and core loss from the rolling direction (0.degree.) to the transverse direction (90.degree.). In grain oriented steel, the rolling direction coincides with the easily magnetizable &lt;001&gt; crystal axes and the grains in the steel occupy a very sharp {110}&lt;001&gt; texture. It is generally believed to be desirable for grain oriented steel to have a substantially complete Goss texture. To this end, an average displacement angle of individual grains from the {110}&lt;001&gt; orientation is as small as possible, for example, within 3.degree..
A typical process for making grain oriented silicon steel generally includes hot rolling a high alloy steel, containing about 3% or more by weight of silicon. The steel is then solution annealed to dissolve second phase particles and is closely control cooled to produce fine second phase precipitates. Next, there is a two-stage cold reduction, with an intermediate annealing operation. The cold rolled sheets are then primarily recrystallized in a decarburizing atmosphere to remove particles that inhibit grain growth. Secondary recrystallization is then employed in order to grow very large grains (&gt;5 millimeters) possessing the Goss texture. For example, see U.S. Pat. No. 5,342,454 to Hayakawa et al.
One disadvantage of grain oriented silicon steels is that they are expensive to manufacture. Grain oriented steel processing typically requires several costly rolling and annealing steps to produce the Goss texture. Moreover, grain oriented steel processing typically requires the use of a continuous annealing facility.
Another disadvantage of grain oriented steel is that it has poor magnetic properties off-angle from the rolling direction in the plane of the strip. In grain oriented steels, permeability is about 28000 G/Oe in the rolling direction (0.degree.) and only about 500 G/Oe in the transverse direction (90.degree.). See the brochure, Armco Oriented Electrical Steels, copyright 1974, Armco Steel Corporation, pages 14 and 36, which is incorporated by reference herein, for typical permeabilities and core losses for grain oriented steel in the rolling direction and off-angle from the rolling direction. Grain oriented steel exhibits a very steep drop in permeability even slightly off-angle from the rolling direction. For example, a typical grain oriented steel has a greater than 50% reduction in permeability between the permeability in the rolling direction and the permeability at 10.degree. from the rolling direction.
An inconvenience of using grain oriented steel is that the permeability is so high it may create problems in some devices. For example, transformer light ballast manufacturers have indicated that typical grain oriented material is undesirable in fluorescent light ballasts because it causes a humming sound when the device is operated.
Conventional non-oriented cold rolled sheet processing includes the steps of hot rolling, coiling, pickling, optional hot band annealing, cold rolling, batch annealing and temper rolling. The equipment for such non-oriented processing costs much less than the equipment for a continuous annealing facility. Non-oriented steel processing often employs compositions that desirably have less silicon than grain oriented steel compositions. However, non-oriented steel has a mostly random distribution of orientations. That is, the magnetically "soft" &lt;001&gt; directions occupy a fairly uniform distribution in space, not only in the plane of the sheet but also pointing into and out of the sheet where they participate only minimally in the magnetization process. As a result, non-oriented steel does not exhibit a significant improvement of magnetic properties in the rolling direction.